1. Field of the Invention
This invention relates to a process for the preparation of block copolymers of the S-B-S type wherein S represents a polystyrene block and B represents a polybutadiene block. More specifically, this invention relates to a process in which hexane is used as the polymerization medium. Still more specifically, this process relates to the use as polymerization medium of a hexane solution of the S-B-S type of block copolymer being produced to give solubility or dispersibility to the initial block of polystyrene.
2. Related Prior Art
Block copolymers of the S-B-S type are known and are considered to be of great utility as self-curing elastomers or as modifiers for diene elastomers. These have a central core of polybutadiene and two terminal blocks of polystyrene. However, in view of the greater insolubility or non-dispersibility of the initial polystyrene block, a solvent such as an aromatic hydrocarbon or cyclohexane has been used in its preparation.
This problem is recognized in British Pat. No. 1,130,770 which states on page 2, lines 60-65, "In each of these processes, however, a vinyl aromatic hydrocarbon is required to be first polymerized and therefore the polymerization solvent used must be wholly or mainly a naphthenic or aromatic hydrocarbon solvent".
Similar recognition of this solubility problem is indicated in U.S. Pat. Nos. 3,231,635, 3,265,765 and 3,427,364 wherein statements are made regarding the necessity to use aromatics or naphthenic solvents or mixtures of these with other solvents.
Hayes U.S. Pat. No. 3,402,160 describes the suspension polymerization of styrene in hexane to form polystyrene whereby a soluble diene or diene-styrene random copolymer having at least 50% diene is used as a suspension agent. No mention is made of S-B-S type block copolymers nor of their production by such a process. The patentee describes the improvement in impact strength of the polystyrene resulting from the embodiment of the diene polymer suspension agent in the polymer product. In this case the resulting impurity in the product produces a desirable result. In cases where patentee's type of suspension agent is undesired in the final product or produces undesirable results therein, patentee's process could be unsatisfactory.
Because of its lower boiling point and the resultant energy savings in the use of hexane as compared to benzene and cyclohexane, it is desirable to have a process designed to permit the use of hexane in preparing the S-B-S block copolymers without modification of the properties of the product by the introduction of foreign types of materials used as suspension agents. Moreover, the use of a process conducted in hexane makes it adaptable for use in equipment designed for the preparation of polybutadiene in hexane and for the recovery of hexane therefrom.